The strictly controlled experiments saw mice divided into six groups, including a control group that drank only water. The ones fed with grapefruit juice got a mixture diluted with water at different concentrations.

The mice were fed a diet that was either 60% fat or 10% fat for 100 days.

Mice that ate the high-fat diet and drank diluted grapefruit juice not only gained less weight than their control counterparts, they also had a 13 to 17% decrease in blood glucose levels and a threefold decrease in insulin levels.

The researchers gave one group of mice naringin, a bioactive compound in grapefruit juice that has been identified as a key agent in weight loss, and another group metformin, a glucose-lowering drug.

"The grapefruit juice lowered blood glucose to the same degree as metformin," said Joseph Napoli, professor and chair of nutritional sciences and toxicology at UC Berkeley.

The group of high-fat-diet mice that received naringin had lower blood glucose levels than the control group, but there was no effect on weight, suggesting that some other ingredient in grapefruit juice is also beneficial.

Among the mice on a low-fat diet, those that drank the grapefruit juice saw a two-fold decrease in insulin levels, but there was no significant change in weight or other metabolic variables.

Typical factors for weight loss like ingested calories, temperature and level of activity was comparable among the various groups, ruling out any other reason than the grapefruit juice.

Researchers said they hoped to explore in more depth the link between grapefruit and health benefits.